How can I change a text from the button texture?

I write my first 2D game and currently I'm trying to finish a menu level. I downloaded a pack of buttons from this site: The problem is, I have no idea how to change the text which is placed on the button. All the given buttons have a "Button" label on them, and I would like to change it to "Play", "Options" etc. Shall I do it with Gimp or is there any easier and better way? I would love to keep the fonts related to the buttons because imo they look great. I couldn't find an answer to this problem though it is likely not complex at all. I hope you can help me!